First of all in a pre-amp what exactly do the 12at7 and the 12ax7 tubes do? I know that each type of tube has a particular function, but not exactly sure what the function of each is.

Second question, I understand that both E-H and Sovtex are both owned by the New Sensor Corp but what wondering from a sonic perspective what are differences between the E-H and Sovtex tubes (12at7 and 12ax7). At present I'm running NOS Sylvanias or NOS RCA 12at7 and Sovtex 12ax7 in my amp (Jolida SJ-502A) but down the line I thinking of getting away from NOS and going to current production stock simply because of availability (E-H/Sovtex tubes seem very available at the local music shops where I live).

Bottom line, I trying to educate myself on tubes (and it can get a little confusing).

Check out the following websites, they contain good information regarding every aspect of tubes/tube equipment. You might be able to find the answers there.http://www.audioasylum.com/cgi/d.pl?audio/faq.html (scroll down to the section on tubes.http://www.head-fi.org/forums/showthread.php?t=20676

I don't have any experience with 12xxx tubes, but I own both the EH and Sovtek 6SN7s, and I can tell you that they are definitely not the same tube, either in construction or sound. In my Wright WLA12A the EHs are very superior to the Sovteks, and close enough to NOS tubes I've tried that I don't worry about whether the supply of NOS gets too expensive. The Sovteks sound pretty bad in comparison, grainy, constricted, no bass. If only Sovtek existed and not EH, I'd be hoarding NOS 6SN7s whenever I could.

New Sensor is the US importer, and they name-brand these tube lines for this market. Both lines are manufactured in Russia in the Reflektor factory (as are some other brands imported by New Sensor). I'm not sure this applies in every case, but for the types I've seen that are offered in both lines, the Sovtek and EH tubes are actually physically distinct from one another. But personally, when it comes to new-manufacture miniature 9-pin, 12-volt dual-triode varieties (such as 12AX7 and 12AT7) in specific, my preference has usually been for the Yugoslovian EI make. (Also over the SED, JJ and Chinese versions I've heard.) In my experience good examples of the EI can sound almost or equally as good for less money than some of the moderately-priced varieties of NOS tubes, although not as good as the better varieties of NOS tubes (which of course can cost many times their price). But despite that they're brand new and seem acceptably durable in use, you're faced with a high percentage of noisy or otherwise sub-par tubes straight from the production runs (not saying this is unique to EI), even after the manufacturer's and/or importer's testing and QC, so it's still a good idea to deal with a reseller who'll test, grade, match and guarantee their tubes' performance, especially for high-gain preamp/phono stages. It's also often worthwhile to try some inexpensive tube-damping polymer rings with these types.

I know you don't want to hear this, but if you really want to know the difference between the EH's and Sovteks, buy a set of each and try them. It's the only real way of determining what "you" like, not what "others" may like.For example, I personally love the EH 12ax7's. I think they are equal to if not better than most NOS including Telefunkens. Others may dissagree, that's why you have to find out for yourself!!!